The most common carbonized pitch articles include pitch moldings and pitch fibers. Carbonized pitch moldings are generally known, and typically consist of a carbon filler material and a carbon “matrix” or “binder” material. The carbon filler material is generally carbon or graphite fibers, carbon or graphite particles, or a combination thereof, while the carbon binder material can be provided by a pitch, an organic resin, the thermopyrolysis of a carbon-bearing vapor, or combinations thereof. During carbonization of a mixture of such filler and binder materials, the binder material becomes interbonded with the carbon filler material. The binder material interbonds with the carbon filler material.
A general method for producing carbon pitch moldings, in other words particulate carbon composite moldings, includes the mixing of carbon filler material and carbon binder material, with the subsequent heat treatment of this mixture in an inert atmosphere, at elevated temperatures of at least about 500° C., to decompose the binder material. This heat treatment, commonly referred to as “carbonization,” leaves behind a carbonaceous residue that is bonded to the carbon filler material.
Carbon pitch fibers are also generally known and are typically comprised of mesophase pitches. Pitch-based carbon fibers are generally produced by first either obtaining mesophase pitch commercially or producing mesophase pitch through heat-soaking of an isotropic pitch. Then, this mesophase material is melt-spun, or otherwise processed to form pitch fibers. The pitch fibers are oxidized in air to form an infusible structure, and thereafter carbonized, under inert atmosphere and high temperatures, to produce a carbon fiber, or they may be graphitized at even higher temperatures to produce a graphite fiber. Such fibers are used in many applications, and can also undergo surface treatment to affect adhesion in a resin matrix for carbon molding applications.
The strength and other mechanical properties of carbonized pitch moldings suffer from the fact that pitch moldings tend to have a relatively high degree of porosity. This porosity results during carbonization of the mixture of the carbon filler material and carbon binder material due to bloating of the pitch molding from the volatilization of low molecular weight components and the removal of gaseous heteroatoms, such as oxygen and nitrogen, within the binding and filler materials. For pitch filler and binder materials without stabilization, up to about 60% by weight of the pitch may be lost during carbonization under inert environment, as a result of volatilization and gasification. This loss of material results in the creation of voids within the carbonized pitch molding, producing a composite having high porosity, low density, and reduced strength.
Carbonized pitch moldings also suffer from a lack of homogeneous stabilization throughout the molding, especially in those moldings having larger, on the order of at least 70 μm, thickness. Typically, during carbonization of thick pitch moldings, the bloating phenomenon is observed, leading to discarded moldings. Others have attempted to stabilize bulk pitch articles, but, due to the limited depth of oxygen diffusion in such articles, these attempts have not been very successful at achieving full stabilization without the creation of micropores. If one prepares a pitch molding first and subsequently tries to stabilize it in oxygen rich environment, full stabilization throughout the body is impossible.
During stabilization of pitch fibers, low molecular weight volatile components within the pitch are evaporated and, therefore, a skin/core microstructure tends to be produced upon carbonization. Upon carbonization, the outer portions of pitch fiber generally exhibit an onion-skin structure characterized by fine sheets of crystallites, while the core portion tends to exhibit a more random structure. Such a microstructure does not promote good mechanical properties.
Although the prior art has attempted to deal with these problems, as evidenced by the patents and literature referenced herein, the present invention provides carbonized pitch articles and methods for producing the same heretofore unknown in the art. Particularly, this invention teaches mixing synthetic mesophase pitch powder with partially anisotropic pitch prepared, at least in part, from petroleum-derived isotropic pitch, forming an article from the mixture, and, subsequently carbonizing the article in an inert environment. Particularly, such articles include pitch fibers and pitch moldings. The present invention also teaches an improved preparation of partially anisotropic pitch, wherein a mixture of synthetic mesophase pitch and petroleum-derived isotropic pitch is thermally polymerized by heat-soaking.